I have an asus mobo Prime z390m- plus and a gtx 1070 dual 8gb, I installed a rajintek air cooler on my GPU with 2 EK fans (a lot quieter and cooler), the EK adaptor for fans to GPU doesn't work in my case.
But i found a way around with fan xpert 4 from the aisuite asus.
I plugged my 2 GPU fans on the a header 2 on my mobo and can tell fan xpert 4 to use my fan curve for this header using the gpu temp.
there is 2 little issues: 1st it works only with msi afterburner (tried gpu tweak and it doesn't work). 2nd issue (probably the most annoying) every time I start my start or restart my pc, I have to go in fan expert 4 on the fan 2 (gpu) and go in source, click gpu 3 times (deselect and select again, why 3 times i don't know) then apply and it works well.
in my case fan start spinning at 35c and follow the fan curve.
I was also able to set up my header 1 for air extraction as cpu and/or gpu source, they will follow the fan curve I set up to either the hottest of both.
It's reliable as long as I reactivate gpu temp tracking every time I start my pc.
I'm not fully happy with that and keep looking for a better option, hence how I found this post